Electromagnetic contactors are known and have been used for a long time, for example, as a switching means between a voltage source and an electric motor.
From, for example, patent publications FR-B1-2 601 191, FR-B1-2 617 634 and WO-A1-86/01332 it is known to arrange a control means, preferably a switching transistor, in series with the operating coil of the contactor and to control, with the aid of the control means, the mean value of the voltage applied to the operating coil such that the'current of the coil is maintained at a desired value. In this way, the function of the contactor may become, to a certain extent, independent of the supply voltage, the supply voltage can then vary between the limits U.sub.min -U.sub.max, e.g., 80-275 V, where U.sub.min is the minimum required voltage for closing the contactor. Further, by using a higher reference value for the current during the closing operation of the contactor and a lower reference value in the closed condition, a rapid closing operation may be obtained simultaneously with low power consumption in the closed condition.
In a contactor, the inductance of the operating coil is changed during the closing operation because of the movement of the armature. This change of inductance causes an electromotive force (emf)to form in the operating coil. This emf is proportional to the time rate of change of the inductance and is directed opposite to the voltage applied to the coil. In this way, in contactors without control of the coil current, when the armature has reached a high speed, a considerable reduction of the resultant voltage is obtained and hence there is a reduction in the current of the coil and in the acceleration of the armature during the latter part of the closing operation.
In a contactor with control of the coil current, however, the control system will sense the current which decreases during the closing operation and will attempt to counteract this by increasing the voltage applied to the coil. This results in the coil current and hence the acceleration of the armature, generally becoming considerably higher during the latter part of the closing operation than is the case in a corresponding contactor without current control. This, in turn, results in a high speed of the armature at the end of the closing operation. It has been found that this "hard" closing operation entails significant drawbacks. Increased wear occurs on the pole surfaces, which results in an increased risk of functional disorders, for example by a so-called remanence air gap decreasing or disappearing and causing adherence of the armature. Further, the risk of functional disorders increases in that there are high mechanical stresses on all the moving parts of the contactor. An additional disadvantage is the increasing tendency of contact bouncing.